The Spirit V1.8.x code base has been integrated with Spirit V2. It is now
called Spirit.Classic.
Since the directory structure has changed (the Spirit Classic headers are
now moved to the $BOOST_ROOT/boost/spirit/home/classic directory), we created forwarding headers allowing existing
applications to compile without any change. However, these forwarding headers
are deprecated, which will result in corresponding warnings generated for
each of the headers starting with Boost V1.38. The forwarding headers are
expected to be removed in the future.

The recommended way of using Spirit Classic now is to include header files
from the directory $BOOST_ROOT/boost/spirit/include. All Spirit Classic headers in this directory have 'classic_'
prefixed to their name. For example the include

#include<boost/spirit/core/core.hpp>

now should be written as:

#include<boost/spirit/include/classic_core.hpp>

To avoid namespace conflicts with the new Spirit V2 library we moved Spirit
Classic into the namespace boost::spirit::classic.
All references to the former namespace boost::spirit
need to be adjusted as soon as the header names are corrected as described
above. As an alternative you can define the preprocessor constant BOOST_SPIRIT_USE_OLD_NAMESPACE, which will
force the Spirit Classic code to be in the namespace boost::spirit
as before. This is not recommended, though, as it may result in naming clashes.

The change of the namespace will be automatically deactivated whenever the
deprecated include files are being used. This ensures full backwards compatibility
for existing applications.